It is often desirable for wireless communications devices to operate in a full-duplex mode of operation in which radio frequency (RF) signals are simultaneously transmitted and received. Further, it is often desirable for wireless communications devices to operate in a carrier aggregation mode of operation in which RF signals are simultaneously transmitted and/or received over multiple RF frequency bands. Supporting full-duplex and carrier aggregation modes often requires specialized RF filtering circuitry for appropriately separating and combining various RF signals. FIG. 1 shows conventional RF front end circuitry 10 capable of both full-duplex and carrier aggregation modes of operation.
The conventional RF front end circuitry 10 includes an antenna 12, a diplexer 14, a first duplexer 16A, and a second duplexer 16B. The diplexer 14 includes a diplexer common node 18A, a first diplexer input/output node 18B, and a second diplexer input/output node 18C. The diplexer common node 18A is coupled to the antenna 12. The first diplexer input/output node 18B is coupled to the first duplexer 16A. The second diplexer input/output node 18C is coupled to the second duplexer 16B. The diplexer 14 is configured to pass RF transmit signals and RF receive signals within a first RF frequency band between the diplexer common node 18A and the first diplexer input/output node 18B while attenuating signals outside of the first RF frequency band in this signal path. Further, the diplexer 14 is configured to pass RF transmit signals and RF receive signals within a second RF frequency band between the diplexer common node 18A and the second diplexer input/output node 18C while attenuating signals outside of the second RF frequency band in this signal path. Accordingly, those skilled in the art will appreciate that the diplexer 14 enables the simultaneous transmission and/or reception of RF signals within the first RF frequency band and the second RF frequency band (i.e., carrier aggregation).
The first duplexer 16A includes a first duplexer common node 20A, a first duplexer transmit signal node 20B, and a first duplexer receive signal node 20C. The first duplexer common node 20A is coupled to the first diplexer input/output node 18B of the diplexer 14. The first duplexer 16A is configured to pass RF transmit signals within the first RF frequency band from the first duplexer transmit signal node 20B to the first duplexer common node 20A while attenuating other signals. Further, the first duplexer 16A is configured to pass RF receive signals from the first duplexer common node 20A to the first duplexer receive signal node 20C while attenuating other signals. Accordingly, those skilled in the art will appreciate that the first duplexer 16A enables the simultaneous transmission and reception of RF signals within the first RF frequency band (i.e., full duplex).
The second duplexer 16B includes a second duplexer common node 22A, a second duplexer transmit signal node 22B, and a second duplexer receive signal node 22C. The second duplexer common node 22A is coupled to the second diplexer input/output node 18C. The second duplexer 16B is configured to pass RF transmit signals within the second RF frequency band from the second duplexer transmit signal node 22B to the second duplexer common node 22A while attenuating other signals. Further, the second duplexer 16B is configured to pass RF receive signals within the second RF frequency band between the second duplexer common node 22A and the second duplexer receive signal node 22C. Accordingly, those skilled in the art will appreciate that the second duplexer 16B enables the simultaneous transmission and reception of RF signals within the second RF frequency band (i.e., full duplex).
The first duplexer 16A and the second duplexer 16B must be designed in order to appropriately isolate RF transmit signals from RF receive signals in the first RF frequency band and the second RF frequency band, respectively. FIG. 2 shows a conventional duplexer 24 that may be used for the first duplexer 16A and the second duplexer 16B. The conventional duplexer 24 includes a duplexer common node 26A, a duplexer transmit signal node 26B, and a duplexer receive signal node 26C. Transmit signal filtering circuitry 28 is coupled in series with a transmit signal path coupler inductor L_CTX between the duplexer transmit signal node 26B and the duplexer common node 26A, such that the transmit signal path coupler inductor L_CTX is coupled to the duplexer transmit signal node 26B and the transmit signal filtering circuitry 28 is coupled between the transmit signal path coupler inductor L_CTX and the duplexer common node 26A. Receive signal filtering circuitry 30 is coupled in series with a receive signal path coupler inductor L_CRX between the duplexer receive signal node 26C and the duplexer common node 26A such that the receive signal path coupler inductor L_CRX is coupled to the duplexer receive signal node 26C and the receive signal filtering circuitry 30 is coupled between the receive signal path coupler inductor L_CRX and the duplexer common node 26A.
The transmit signal filtering circuitry 28 includes a number of acoustic resonators coupled together in a ladder configuration on a transmit signal filtering circuitry acoustic die 32. Specifically, the transmit signal filtering circuitry acoustic die 32 includes a series transmit signal filter path 34 coupled between the transmit signal path coupler inductor L_CTX and the duplexer common node 26A, and a number of shunt transmit signal filter paths 36 coupled between the series transmit signal filter path 34 and ground. The series transmit signal filter path 34 includes a number of transmit signal series resonators 38 each including an input node and an output node and coupled in series such that an input node of a first one of the transmit signal series resonators 38A is coupled to the transmit signal path coupler inductor L_CTX, an output node of a last one of the transmit signal series resonators 38N is coupled to the duplexer common node 26A, and a connection between each adjacent pair of the transmit signal series resonators 38 provides a series intermediate node 40, such that the number of series intermediate nodes 40 is one less than the number of transmit signal series resonators 38.
Each one of the shunt transmit signal filter paths 36 includes a transmit signal shunt resonator 42 coupled in series with a transmit signal shunt inductor L_SHTX between one of the series intermediate nodes 40 and ground. Specifically, a first transmit signal shunt resonator 42A is coupled in series with a first transmit signal shunt inductor L_SHTX1 between a first series intermediate node 40A and ground, and a last transmit signal shunt resonator 42N is coupled in series with a last transmit signal shunt inductor L_SHTXN between a last series intermediate node 40N and ground.
The receive signal filtering circuitry 30 similarly includes a number of acoustic resonators coupled together in a ladder configuration on a receive signal filtering circuitry acoustic die 44. Specifically, the receive signal filtering circuitry acoustic die 44 includes a series receive signal filter path 46 coupled between the receive signal path coupler inductor L_CRX and the duplexer common node 26A, and a number of shunt receive signal filter paths 48 coupled between the series receive signal filter path 46 and ground. The series receive signal filter path 46 includes a number of receive signal series resonators 50 each including an input node and an output node and coupled in series such that an input node of a first one of the receive signal series resonators 50A is coupled to the duplexer common node 26A, an output node of a last one of the receive signal series resonators 50N is coupled to the receive signal path coupler inductor L_CRX, and a connection between each adjacent pair of the receive signal series resonators 50 provides a series intermediate node 52, such that the number of series intermediate nodes 52 is one less than the number of receive signal series resonators 50.
Each one of the shunt receive signal filter paths 48 includes a receive signal shunt acoustic resonator 54 coupled in series with a receive signal shunt inductor L_SHRX between one of the series intermediate nodes 52 and ground. Specifically, a first receive signal shunt acoustic resonator 54A is coupled in series with a first receive signal shunt inductor L_SHRX1 between a first intermediate node 52A and ground, and a last receive signal shunt acoustic resonator 54N is coupled in series with a last receive signal shunt inductor L_SHRXN between a last intermediate node 52N and ground.
The various components of the transmit signal filtering circuitry 28 are chosen such that a filter response thereof passes RF transmit signals within a desired RF operating band while attenuating other signals. Similarly, the various components of the receive signal filtering circuitry 30 are chosen such that a filter response thereof passes RF receive signals within the same RF operating band while attenuating other signals. Generally, the largest blocker signals for the receive signal filtering circuitry 30 are the RF transmit signals passed by the transmit signal filtering circuitry 28, which may be unintentionally coupled into the signal path of the receive signal filtering circuitry 30 and cause desensitization of downstream receiver circuitry. Accordingly, it is a primary objective of the conventional duplexer 24 to isolate the RF transmit signals from the signal path of the receive signal filtering circuitry 30. In situations in which multiple RF carriers are used to receive a signal, the difficulty of isolating the RF transmit signals from the signal path of the receive signal filtering circuitry 30 may increase, as discussed with respect to FIG. 3.
FIG. 3 is a diagram illustrating a transmit and receive configuration for a wireless communications device in which primary RF transmit signals within a transmit signal frequency band are transmitted and both primary RF receive signals and secondary RF receive signals within a receive signal frequency band are received (i.e., downlink carrier aggregation). The transmit signal frequency band and the receive signal frequency band form an RF operating band. The RF operating band may be, for example, a Long Term Evolution (LTE) operating band. As will be appreciated by those skilled in the art, it is the job of the conventional duplexer 24 to separate the primary RF transmit signals from the primary RF receive signals and the secondary RF receive signals. As shown in FIG. 3, the secondary RF receive signal is closer to the transmit signal frequency band than the primary RF receive signal. Because of this fact, the primary RF transmit signals will disproportionately affect the secondary RF receive signals, as signals become more difficult to separate the closer in frequency they become. Accordingly, the secondary RF receive signals may require additional attenuation with respect to the primary RF transmit signals. Conventional RF filtering circuitry such as the conventional duplexer 24 generally provides uniform attenuation across the entirety of a transmit signal frequency band and a receive signal frequency band. While this attenuation may be increased to reduce the effect of the primary RF transmit signals on the secondary RF receive signals, the additional attenuation comes at the cost of increased insertion loss and may otherwise decrease the performance of the conventional duplexer 24.
In light of the issues identified above, there has been a trend towards providing tunable RF filtering circuitry in which the attenuation provided across the transmit signal frequency band, the receive signal frequency band, or both are adjustable as desired. Such tunable RF filtering circuitry may be useful, for example, in the situation discussed above with respect to FIG. 3, as the attenuation of the RF transmit signals in the receive signal path may be increased when a secondary carrier is used. However, there are serious design challenges associated with creating tunable RF filtering circuitry. Those conventional designs that do provide adjustability of RF filtering circuitry often do so at the cost of increased area, increased insertion loss, degraded out-of-band response, and the like. Accordingly, there is a need for improved RF filtering circuitry, and in particular for RF filtering circuitry with improved attenuation of RF transmit signals in an RF receive path.